1. Field of the Invention
This invention relates to a method of forming an etch barrier from polyamic acid films during the reactive ion etching of substrate surfaces in the manufacture of semiconductor elements.
2. Description of Related Art
The technique of plasma or reactive ion etching (RIE) is well known and commonly used in the manufacture of semiconductor devices.
A mask or a patterned photosensitive resist is formed over a substrate material such as a silicon wafer or metallic substrate, and the non-masked areas of the substrate are etched by contact with an etching gas such as oxygen in an electric field. Removal of the mask or the remaining resist yields a pattern etched into the surface of the substrate.
Many large scale integrated circuit devices employ a thin film of a polyimide or polyamic acid polymer overlaying the substrate. This layer may serve many functions such as a dielectric layer, a passivation layer to separate or overcoat metallurgy, or as a lift off layer as disclosed in U.S. Pat. No. 4,692,205. In many applications, the polyamic acid layer serves as a masking layer for RIE processes and therefore means must be found to enhance the etch resistance of the polyamic acid film surface which is not to be etched during the etching process.
The most common masking technique used to prepare relief images involves the use of a photosensitive resist material which is exposed to a pattern of radiation and developed to yield an image pattern. Radiation sensitive resists are classified as positive or negative acting. When a positive acting resist is exposed to radiation, the exposed parts can be removed selectively because they become soluble in a developing solution with the unexposed parts remaining insoluble. In the case of a negative acting resist, the exposed parts become insoluble while the unexposed parts remain soluble. In general positive acting resists allow better resolution than their negative acting counterparts. Thus, because of their compatibility with miniaturization, the semiconductor industry has tended to prefer positive acting resists for the manufacture of integrated circuits despite the fact that their resistance to plasma etching is generally inferior to negative acting resists. To counteract the problem of poor etch resistance, it is usual to use a relatively thick layer (typically in the range 0.5 to 1 micrometer) of resist in order to compensate for undesirable erosion but, unfortunately, resolution is reduced as the resist thickness increases.
Most prior art attempts to improve resolution involve the treatment of the resist material itself to improve its resistance to plasma gases, i.e., to enhance its barrier or masking properties. For example, U.S. Pat. No. 4,504,574 discloses a method forming a resist mask resistant to plasma etching by first subjecting a lithographically patterned layer of resist on a substrate to a carbon monoxide plasma treatment followed by a more conventional plasma etch.
U.S. Pat. No. 4,430,153 discloses a method of forming an RIE etch barrier involving converting the top layer of an aromatic polyimide to the corresponding silicon containing polyimide, overcoating that layer with a patterned resist, reactive ion etching the exposed surface and reacting the thus exposed surface of the silicon containing polyimide with oxygen reactive ion etching thereby forming a silicon dioxide barrier layer. A somewhat related process is disclosed in U.S. Pat. No. 4,692,205 wherein a silicon-containing polyimide layer is employed as an oxygen etch barrier in a metal lift-off process.
Other techniques for improving etch resistance involve treating substrates with etch resistant ionic materials. For example, U.S. Pat. No. 4,377,734 discloses a method of forming an etched pattern on a substrate such as silicon or aluminum wherein the substrate is first subjected to a patterned ion bombardment with a metal ion, such as tungsten, which form a passivator against subsequent ion etching. U.S. Pat. No. 4,092,209 discloses a method for rendering silicon substantially non-etchable by potassium hydroxide comprising bombarding the surface of the silicon with phosphorous ions.
While these and other techniques for imparting etch resistance to various substrates are suitable for their intended purposes, they are limiting with respect to the specific chemistry involved. For example, the masking processes disclosed in U.S. Pat. Nos. 4,430,153 and 4,692,205 wherein etch resistance is imparted to a silicon-containing polyimide requires the use of an oxygen-containing plasma in the etch step which converts the silicon into the more etch resistant silicon dioxide. Non-oxidizing etchants based on other chemistry, such as halogen gas etchants, would not appear to be useful in such a system. In addition, modification of resist materials prior to etching in order to improve etch resistance may involve a number of additional process steps and/or the use of noxious materials, and may render the resist more difficult to remove from the substrate.